1. Field of the Invention
The present invention relates to a magnetic tape unit of the type which is provided with a read/write head designed to simultaneously access a plurality of tracks on a magnetic tape and with a phase-locked loop (PLL) for subjecting the read-out data to bit synchronization.
2. Description of the Related Art
An example of conventional magnetic tape unit is shown in FIG. 12 of the accompanying drawings. The conventional unit, as illustrated, includes a magnetic read/write head 51, a read/write circuit 52, a PLL 53 and a data formatter 54. The magnetic head 51 reads out data from a magnetic tape and supplies it to the read/write circuit 52. The read/write circuit 52 demodulates the received data and supplies it to the PLL 53 in which the data is subjected to bit synchronization. The bit-synchronized data is then sent to the data formatter 54. The data formatter 54 performs the detection of data format, while also being responsible for data transmission.
The conventional magnetic tape unit has been found disadvantageous in the following point.
Specifically, in the conventional unit, the bit synchronization by the PLL 53 may fail to be performed properly due to unfavorable conditions, such as wear of the magnetic tape or magnetic head 51, adhesion of particulate matter to the head 51, etc. In such an instance, the data transmission speed may vary unacceptably. The conventional tape unit, however, is not provided with any means to detect such abnormal data transmission, and continues to send the data. Thus, disadvantageously, an error will be contained in the read-out data.
The present invention has been proposed under the above-described circumstances, and its object is to provide a magnetic tape unit capable of promptly detecting the failure in performing bit synchronization.
According to a first aspect of the present invention, there is provided a magnetic tape unit which includes a magnetic tape provided with a plurality of tracks, a magnetic head arranged to simultaneously read out data from the tracks on the tape, a phase-locked loop (PLL) arranged to perform bit synchronization of data read out from each of the tracks on the tape, and a detector arranged to detect a desynchronized track by comparing transmission speeds of data read out from the respective tracks on the tape.
Preferably, each of the tracks on the tape may be provided with a unit data region flanked by synchronization codes to perform byte synchronization. The detector may be arranged to compare the transmission speeds with each other during a period for reading out data from the unit data region.
Preferably, the detector may compare any one of the transmission speeds with remaining transmission speeds, so that failure in bit synchronization is determined by majority rule.
Preferably, the magnetic tape unit of the present invention may further include a timer and a desynchronized track recognizer. The timer may start measuring time when a first piece of particular data is detected in the read-out data from a predetermined number of tracks. The desynchronized track recognizer may be arranged to regard a particular track as a bit-desynchronized track when a second piece of particular data is not detected in the read-out data from said particular track within a predetermined time period after the timer is started.
Preferably, each of the tracks on the tape may be provided with burst pattern regions for performing bit synchronization.
Preferably, the magnetic tape unit of the present invention may further include a burst pattern region detector for detecting the burst pattern regions of each track based on the read-out data from said each track.
Further, the magnetic tape unit may include a bit synchronization instructor for supplying an instruction to cause the PLL to perform bit synchronization with respect to a desynchronized track when a burst pattern region is detected by the burst pattern region detector.
Still further, the magnetic tape unit may include a pseudo-data generator arranged to supply to the PLL a piece of pseudo-data for performing bit synchronization with respect to a desynchronized track.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.